Add a configurable ACP runtime registry (or document how to attach a 3rd-party runtime like omp acp)

## Summary

Buzz desktop's runtime registry is hardcoded to 5 entries (`node`, `node-tools`, `codex-acp`, `claude-agent-acp`, `claude-code-acp`). There is no way to attach a third-party ACP-compatible agent such as [oh-my-pi/omp](https://github.com/oh-my-pi/pi) (`omp acp`) through the UI today, even though the underlying plumbing (`acp_command="buzz-acp"`, `agent_command=""`, `agent_args=[...]`) is generic and would accept any bin name on `PATH`.

## Evidence

String-scan of `D:\Apps\Buzz\buzz-desktop.exe` at offset **55350058**:

```
…
BUZZ_AGENT_PROVIDER…
Buzzruntimesnodenode-toolscodex-acpclaude-agent-acpclaude-code-acp
npmwin-x64binv24.11.0node.exe…
```

That comma-joined tail after `Buzzruntimes` is the runtime registry. The same 5 names appear nowhere else in any of the 5 binaries in `D:\Apps\Buzz\`. A direct `Buffer`-level grep across `buzz-desktop.exe`, `buzz-acp.exe`, `buzz-agent.exe`, `buzz.exe`, `buzz-dev-mcp.exe` for `omp-acp`, `oh-my-pi`, `minimax`, `agentclientprotocol/omp`, `omx-acp` returns **zero hits**.

I built and globally installed a working bridge as a sanity check:

- npm package: `@agentclientprotocol/omp-acp` (local, Apache-2.0)
- bin name: `omp-acp` (sits next to `codex-acp`, `claude-code-acp`)
- implementation: `child_process.spawn("omp.cmd", ["acp", ...argv], { stdio: "inherit" })` — a thin stdio proxy, ~50 lines, no protocol translation because `omp acp` already speaks Agent Client Protocol over stdio (verified via direct `initialize` handshake: `protocolVersion=1, agentInfo={name:"oh-my-pi", version:"17.1.7"}` with full `loadSession`/`mcpCapabilities`/`sessionCapabilities`).

After installing it, I appended a fresh persona to `managed-agents.json`:

```json
{
"name": "OMP-Probe",
"display_name": "OMP-Probe",
"slug": "omp:probe",
"acp_command": "buzz-acp",
"agent_command": "omp-acp",
"agent_args": [],
"is_active": true,
...
}
```

Restarting Buzz desktop preserved `agent_command: "omp-acp"` verbatim — so the file-level path works. Only the UI runtime picker is closed.

## What I am asking for

One of the following, in order of preference:

1. **Make the runtime registry configurable.** Move the 5-name list out of the binary into something user-editable — config file, `BUZZ_RUNTIMES` env var, or a runtime-discoverable convention (any bin on `PATH` matching `-acp` whose path resolves). I would expect this to be a small change touching `crates/buzz-desktop/` near where `Buzzruntimes` lives.
2. **Document the supported runtimes + add `omp-acp`** to the same list. Less flexible for the long tail but unblocks the immediate use case (`goose`'s env contract is the source of the broken-API-key reports, see #3395 — giving users an alternative runtime that doesn't need the bundled provider-key path would also help there).
3. At minimum: a per-persona `custom_runtime` field whose `agent_command` is honored without going through the registry (effectively what I have working today via direct `managed-agents.json` editing, but it currently has no UI path and is not documented).

## Reproduction (Windows 11, `D:\Apps\Buzz\`, omp 17.1.7 installed)

```bash
# 1. install the bridge anywhere on disk
mkdir omp-acp-adapter && cd omp-acp-adapter
# omp-acp.js : transparent stdio proxy to `omp acp`
npm install -g .

# 2. confirm bin on PATH and ACP handshake
where omp-acp
echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":2,"clientInfo":{"name":"probe","version":"0.0.1"}}}' \
| omp-acp
# -> {"jsonrpc":"2.0","id":1,"result":{"protocolVersion":1,"agentInfo":{"name":"oh-my-pi","version":"17.1.7"},...}}

# 3. open Buzz desktop, click "connect", look at the runtime list
# -> node, node-tools, codex-acp, claude-agent-acp, claude-code-acp
# no omp-acp, no way to add one
```

## Environment

- Buzz `D:\Apps\Buzz\`, all 5 binaries from `Jul 25 17:08` / `17:23`
- `omp 17.1.7` already on PATH at `C:\Users\oabdu\AppData\Roaming\npm`
- node 24.11.0, npm 11.6.2

## Related

- #3395 — bundled goose runtime hits 401 because Buzz does not translate `OPENAI_COMPAT_API_KEY` to `OPENAI_API_KEY`. Unblocking an alternative runtime (this issue) does not fix that, but it gives users an escape hatch while it is being triaged.